The present invention pertains to amplifiers and pertains particularly to a common device node balun grounded balanced amplifier.
A common drain amplifier with an ideally grounded drain can be designed that is unconditionally stable. However, in a practical implementation, the length of a bond wire, package lead and so on can result in nonzero drain inductance. The drain inductance introduces instability at various frequencies. The resistance values of the amplifier can be adjusted to re-acquire unconditional stability. However, this adjustment of resistance values can result in a loss of gain and decreased noise performance.
An output balun may take the form of a transformer or, more usually in integrated circuits, a lumped element inductance-capacitance (L-C) ladder structure. In the case of a transformer, the secondary winding has a center tap that is usually grounded. For the ladder type balun, there is a tail node, which is used to ground all of the shunt elements. Herein, the term xe2x80x9cbalun tailxe2x80x9d is used to refer both to the center tap of the transformer and to the tail node of the ladder type balun.
When a balun has an ideal tail ground, amplitude and phase performance is maximized. However, in a practical implementation, the length of a bond wire, package lead and so on can result in nonzero tail inductance. The tail inductance deteriorates the amplitude and phase. The element values of the balun can be adjusted to compensate for tail inductance; however, this results in reduction of useful bandwidth of the balun and costly design iteration.
When an amplifier with drain inductance is integrated with a balun with tail inductance, the effects of the inductances are compounded. The drain inductance can cause the amplifier to be unstable at certain frequencies. In addition there is amplitude and phase deterioration caused by the tail inductance of the balun.
Correction of the stability problem can be accomplished by reducing the gain of the amplifier with additional resistance, resulting in a concomitant increase in noise. Further steps can be taken to improve the drain grounding by use of multiple bond wires and package pins. Additionally, multiple high-grade capacitors are required to bypass the drain node to ground. Phase and amplitude irregularities can be addressed by absorbing the tail inductance into the other component values of the balun, resulting in reduced bandwidth and costly design iteration.
In accordance with the preferred embodiment of the present invention, an amplifier circuit includes an amplifier stage and a balun stage. The amplifier stage includes a common node connected to an external ground. An inductance is located between the common node and the external ground. The balun stage is connected to the amplifier stage. The balun stage includes a balun tail. The balun tail is directly connected to the common node of the amplifier stage so that a resulting connection between the balun tail and the common node bypasses the inductance between the common node and the external ground.